Thermodynamic Potential for the Anodic Dissolution of n‐Type Semiconductors: A Crucial Factor Controlling Durability and Efficiency in Photoelectrochemical Cells and an Important Criterion in the Selection of New Electrode/Electrolyte Systems

We show that the standard potential for anodic dissolution, , of n‐type semiconductors (e.g., for is + 0.08V vs SCE) plays a key role in ultimate efficiency of thermodynamically stable n‐type semiconductor‐based photoelectrochemical cells. The selection of redox active substances that can be used for competitive capture of photogenerated holes is limited to those systems where the product does not have the potential to oxidize the semiconductor. Where and represent the position of the valence and conduction band positions at the interface, respectively, we must conclude that is at a more negative potential (vs. a reference) than , if the semiconductor undergoes photoanodic dissolution. Quenching of the photoanodic dissolution by competitive hole capture by some electrolyte component, say A, is possible if lies at a potential more negative than . But additionally, if the A+, the oxidation product, is to be incapable of oxidizing the semiconductor, must be more negative than. Consequently, for the semiconductor to be thermodynamically stable in the A/A+ electrolyte the maximum photovoltage output, , can be no greater than for more negative than Naturally, kinetic inertness of the semiconductor to an oxidant with more positive than may allow its presence and/or use in a more efficient cell. N‐type and semiconductors in aqueous electrolytes are treated in detail. Some preliminary comments are made concerning p‐type materials.